Add DESIGN.md for Python RST collage engine

Co-authored-by: Cursor <cursoragent@cursor.com>

DESIGN.md

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+# DESIGN — Fractal mosaic Python engine (Tower)
+
+## Goal
+
+Headless RST collage generator for the Tower mosaik inbox, adapted from Seth Benton’s Fractal-Mosaics (GPL).
+
+## Pipeline
+
+1. Load tile pool; cache mean RGB + HSV histogram (CBIR prefilter).
+2. Resize master to profile `master_max`.
+3. For each tile size (largest first), scan uncovered centers.
+4. Rank tiles by histogram/mean distance (**selection only — no recolor**).
+5. Estimate rotation/scale via log-polar phase correlation; snap to discrete rot/scale candidates.
+6. Place with edge alpha blend if RMS and overlap pass thresholds.
+7. Write PNG.
+
+## Profiles
+
+See `src/fractal_mosaic/profiles.py`: `wenig`, `mittel`, `dicht`.
+
+## Non-goals (v1)
+
+- Faithful bit-identical MATLAB port
+- Color transfer / hue variants
+- GPU acceleration